FIG. 1 schematically shows the structure of a typical conventional apparatus implanting an ion microbeam. In FIG. 1, ions emitted from an ion source 1 are accelerated and focused onto a sample 3 by a pair of electrostatic lenses 2, 2'. An EXB (Wien) filter 4 is placed between the two lenses 2 and 2', so that the desired ion seeds can only travel straight through the Wien filter 4, and pass through an iris 6. FIG. 2 shows the Wien filter 4 in detail. In the Wien filter 4, a uniform electric field E is produced by parallel electrodes 10, 11, and a uniform magnetic field B is produced by magnetic poles 12, 13. When monovalent ions of a mass M are accelerated at an acceleration voltage V.sub.0 and travel straight through the filter 4 to form an ion beam 14, the following is the relationship between the mass M, acceleration voltage V.sub.0, uniform electric field E, and uniform magnetic field B: EQU (2eV.sub.0 /M).sup.1/2 =E/B (1)
where e is a unit electric charge.
Only those ions which have a mass that satisfies Equation (1) are able to pass through the Wien filter. Ions lighter than these ions which are travelling straight travel along a light path 15, and heavier ions travel along a light path 16, so that, none of these ions are able to pass through the iris 6. The focused beam 7 is deflected by a beam deflector 8 to scan the sample 3. In the apparatus of FIG. 1, since the ion optical axis is straight, neutral particles emitted from the ion source 1 are not affected by electromagnetic field of the beam-focusing system, reach the sample 3 in the form of a diverging beam 9, as indicated by the broken lines, and contaminate the sample 3. The degree of sample contamination can be calculated roughtly as described below. In an apparatus for implanting an ion microbeam, a liquid metal ion source is usually used as the ion source 1. A single metal or an alloy containing the desired ion seeds is employed as the ion material, it is heated to a temperature above its melting point in the ion source 1 so that it melts. During this time, a quantity of neutral particles corresponding to the vapor pressure of the molten metal are emitted from the end of the ion emitter, and reach the sample 3.
If the temperature of the molten metal is denoted by T[.degree.K.], the vapor pressure at this temperature by P [Torr], the atomic weight of the emitted neutral particles by M [amu], the area of the vaporization region at the end of the ion emitter by S [cm.sup.2 ], and the distance from the end of the emitter to the sample 3 by L [cm], the degree of contamination N [atoms/cm.sup.2.sec] of the sample 3 by the vaporized particles, is given by the relationship: EQU N=3.51.times.10.sup.22 [P/(MT).sup.1/2 ][S/.pi.L.sup.2 ].a (2)
where a is the probability that vaporized particles will adhere to the surface of the sample, and is approximately 1.
For instance, with a liquid metal ion source using gold as the ion material, the melting point Tm is 1336.degree. K., and the vapor pressure P is 10.sup.-4 Torr at a temperature of T=1413.degree. K. Gold has an atomic weight of M=197 amu. If the area is S=1.times.10.sup.-2 cm.sup.2 and the distance is L=40 cm, Equation (1) shows that the degree of contamination N is 1.33.times.10.sup.10 atoms/cm.sup.2.sec. If converted into a current density, this value becomes 2.1.times.10.sup.-9 A/cm.sup.2.
However, this value only concerns atoms vaporized from the ion source 1 as neutral particles, it does not include the neutral particles that are generated by the conversion of electric charge as emitted ions are travelling.
The sample is particularly contaminated when a high concentration of ions is implanted, i.e., when the beam is irradiated for an extended period of time, when a material with a high vapor pressure is used as the ion material in the ion source 1, or when there are impurities therein.